CosmoMC seems to be running for omegach2=0.04 at the moment, though I'm hoping to get it to run with omegach2=0. This might be because I should be assigning a prior to more parameters? Please advise, thanks.

I expect for the specified theta values it cannot solve for a value of H0 in the allowed range once you set omch2=0. Make sure you start with parameters that correspond to non-ridiculous H0 and theta values.